USGS: CA storms dropped less rain than expected

JOHN ANTCZAK, Associated Press Writer

Southern California foothill communities escaped potentially disastrous debris flows from fire-scarred mountains during last week's storms because total rainfall was far less than expected, the U.S. Geological Survey said Wednesday.

Areas burned by a 250-square-mile summer wildfire in the San Gabriel Mountains received totals of 4 1/2 inches to 8 1/2 inches of rain - significantly less than the forecast of up to 16 inches, the USGS said.

More than 2,000 homes in foothill communities northeast of Los Angeles were ordered evacuated during the storms, but many people did not comply, and there were complaints that evacuations were unnecessary.

USGS geologist Susan Cannon, who studies debris flows that follow wildfires in the West, said there likely would have been property damage or life-threatening situations if the area had gotten the rain that was forecast.

With the rainy season only halfway over, "the hazard hasn't gone away," she said, urging people to obey any evacuation orders to stay safe.

The USGS describes debris flows as one of the most dangerous of geologic phenomena - rivers of mud, rocks and other debris many feet deep that surge down mountains with little warning at speeds up to 35 mph.

In an October emergency assessment of danger to neighborhoods below the face of the San Gabriels, Cannon and colleagues noted that in Southern California, post-fire debris flows can be caused both by short-duration, high-intensity thunderstorms and by longer duration, lower intensity storms.

The USGS said last week's storms came in a series of pulses. The first, on Jan. 18, was as intense as forecast.

"They were spot-on with the rainfall totals and intensities," Cannon said. "There were debris flows in many, many canyons."

The flows were intercepted by debris basins designed to protect neighborhoods below, but they also reduced the capacity of some basins and left them susceptible to overflowing in subsequent storms.

On Jan. 20, forecasts called for 4 to 6 inches of rain in the area in a short period. Instead, rainfall amounted to 1 to 2 inches over a prolonged period, the USGS said.

As the day developed, "we could see the high rainfall was still happening but it just wasn't happening along the San Gabriel front," Cannon said.

Indeed, the storms caused damaging flooding in some low-lying urban areas including the port communities of Long Beach and San Pedro in south Los Angeles County, while foothill homes were mostly unscathed.

Some foothill residents who refused to evacuate cited spurts of green appearing on the blackened mountains in the months since the fire, but Cannon said it takes significant regrowth to have an effect.

"Green is a good sign, but there's not enough up there to make a difference," she said.

One of the worst post-fire debris flows to hit the foothills - dozens killed and hundreds of homes destroyed - occurred in 1934, too long ago for most people to know about.

But Cannon cited a few of the more recent tragedies, including a 1978 debris flow that killed 13 people as it wiped out the community of Hidden Springs in a canyon of the San Gabriels.

She said she wished there was time to show people the scale of debris flows by visiting the scenes where 16 adults and children were killed in the San Bernardino Mountains east of Los Angeles in 2003. Some of the bodies were found miles downstream.